This invention relates to a silent operating radar, i.e., a radar that detects a target and its position without the countermeasure operator located at the target being able to detect the presence of the radar.
More specifically, this invention relates to a silent operating radar that provides transmitted signals with characteristics which allow optimum processing by the radar while taking advantage of limitations inherent in typical radar interception receiving equipment located at the target.
Today, the problems of modernized warfare have increased to the point that high speed aircraft or other craft well-equipped with newly developed electronic countermeasure receivers and jammers can quickly penetrate areas to be defended. Prior to this invention, invading aircraft or other craft have had the benefit of knowing that they were being detected because of the large signal power levels utilized in the tracking radar. Once they determined that they were in fact being observed, they would enter evasive patterns and also utilize their countermeasure electronics to jam the tracking radar. Furthermore, they would be able to launch missiles to home in the radar signal and thereby cause the radar""s destruction. This advantage that the target aircraft has had is removed by the usage of a radar system employing the invention to be described. With the utilization of the invention the invading craft can no longer determine that their presence has been detected. Not knowing that they have been detected, the invading craft does not take evasive measures. He can not turn on his electronic countermeasure equipment to mislead the radar nor can he turn on his jamming equipment since these would give away his presence. Jamming and ECM equipment is only employed by the hostile craft only after he has been detected. Again, since the invader has not detected the presence of a radar, he has no reason to attempt to utilize radar homing missiles to destroy the radar.
The ability to operate a radar system in a silent mode is similarly important where ocean going vessels are involved since the presence and location of a moving fleet or submarine is critical to the success of an invading aircraft. As has been mentioned, once the invading aircraft is aware that it is being observed it can rely on its countermeasures equipment to mask its actual position. This advantage is present whether the tracking radar is ground-based, flying, or on ocean-going vessel. While the range of silent operation may be lessened by the size of the tracking radar""s host craft and related antenna, the principle of this invention applies equally as well to them as with ground-based radars. Furthermore, the invention could also be utilized against search ships as well as aircraft.
The remainder of the description will be related to a ground-based radar but it should be understood that the invention is not limited to a ground-based environment but can be utilized in ships or aircraft having the capacity to handle such equipment.
The invention to be described more fully hereafter removes completely the possibility that the oncoming craft will be able to detect the presence of an operating ground-based radar. This has been accomplished through the utilization of equipment providing transmission signals with specific characteristics that allow optimum processing while taking advantage of limitations inherent in typical signal receiving equipment. In addition, the ground-based radar utilizing the concept of the invention is optimized to efficiently detect and process a target""s reflected signals.
It is well known that the receivers at the target making an approach have only to overcome a one-way path loss with reference to the transmitted radar signal, while the ground-based radar has the additional loss inherent in the reflected radar signal from the incoming target. This reflected loss is measured as the fourth power of the radial distance from the target to the ground-based radar, while the one-way path loss to the target is on the order of a second power loss based on the radial distance from the radar to the target. Since the ground-based radar receives what is in essence a two-way path loss, the ground-based radar must be optimized to a point at which the advantage held by the target because of path loss considerations is overcome.
This invention provides a range of self-protection around the ground-based radars. This self-protection range may be defined as an intermediate range within which radar intercept receiver systems having set operating characteristics cannot detect the presence of the ground-based radar system. This aforementioned self-protection comes about due to the ground-based radar""s detection advantage over the approaching target.
Where high energy per cycle is transmitted the target receiver operator can detect the presence of the ground-based radar by slowly sweeping the bandwidth until the strong signal is identified. In this type of strong transmitted signal environment the transmitted signal only experiences a one-way path loss while a return echo from the plane experiences a second path loss, previously noted. If the ground-based radar has the same receiver capabilities as the target""s receiver, the target could detect, the fact that it was being searched for before the ground-based radar could detect the target""s presence.
The invention to be described makes use of transmitted low energy per cycle signals which take the form of wide-band, purely random noise. In addition the ground-based radar, in accordance with the invention, has optimized all radar reception advantages to a point where even an extremely weak return signal can be detected though buried in the wide band noise originally transmitted and then reflected, whereas the target radar, because of space limitations, the size of the antenna and weight limitations cannot be optimized to have anywhere near the overall sensitivity and capacity to discriminate as a ground-based radar.
With the aforementioned transmission characteristics it becomes apparent that where a low energy signal per cycle is transmitted, there is some point near the ground-based radar where the one-way path loss is insufficient to prevent the target from detecting the low energy signal. Further removed from the ground-based radar, the target cannot detect the presence of the ground-based radar but the transmitted signals are still being reflected by the target. At this point the optimized ground-based radar functions to detect even the very weak reflected signals, experiencing a two-way path loss. There is of course a specific distance through which the reflected signal can pass before it is attenuated to a point where the ground-based radar even with optimized receivers cannot detect the reflected signal. The just mentioned specific distance represents the ground-based radar""s detection advantage over the target. The distance from the near point, where the target can detect the presence of a signal being transmitted, and the specific distance just noted defines the ground-based radar""s region of self-protection. This may be referred to as intermediate self-protection range or the volume that it defines about the ground-based radar may be termed a self-protection shell.
It is therefore apparent that as the energy per cycle changes by way of increase or decrease, the dimensions of the self-protection range or shell will change. The operator of the ground-based radar therefore has the ability to increase or to decrease the range being searched in a manual manner or automatically in a programmed manner to be described in detail more fully hereafter.
The silent ground-based radar involved here will transmit a spread spectrum which is essentially noise and then use coherent integration and correlation detection to extract information from the return echo. The fact that optimum detection is possible is brought about because the radar can maintain a record of the signal it has transmitted long enough to compare the record with the return echo and find a correlation. This detection of the echo return is possible because the ground-based radar has stored, as noted above, the history of the signal prior to its transmission and therefore, when the return echo appears, this signal history is compared by correlation techniques to be described and from it is extracted information concerning the target""s position and velocity.
Utilizing this technique, the radar employs a detection advantage that can be over one million times that of a typical target""s receiving equipment, this advantage being gained from the fact that the target has no prior knowledge of the history of the transmitted radar signal.
In the above description, it has been mentioned that the ground-based radar has a maximized or optimized advantage over that of the target. This optimization of detection advantage is further enhanced by the ground-based radar""s ability to use a much larger antenna than possible on the target aircraft. It can therefore be seen that the capture area of the ground-based radar can provide typically an advantage over the target of a magnitude greater than 1,000 to 1 especially where large ground or ship-based radar antenna-are available.
The ground-based radar""s circuitry and components are similarly maximized to obtain signal-to-noise ratios not obtainable by receiving equipment located on a target due to size, weight, power and other limitations. Along with the ability to maximize antenna capture area there is also the ability to design the antenna for maximum efficiency at the known frequencies that are to be transmitted. As noted above, target aircraft antenna must be designed for wide band reception and must be compatible with flight characteristics of the craft. Therefore it can be seen that the ground-based radar can maintain an advantage of a magnitude many times that of a typical target receiver.
A final and important aspect of optimizing the detection advantage of ground-based radar lies in the conjunctive use of an intercept radar receiver that is capable of finding the most crowded portion of the frequency spectrum and then it can shift the transmission of the ground-based radar into the region of greatest signal density and thereby reduce the probability of the target detecting the transmission of the ground-based radar""s signals.
In addition to its ability to detect targets without itself being detected, silent radar has certain other features which increase its useful-ness as described below:
D-1931 Discrimination Against Clutter
Since silent radar will employ both range and speed gating, it has two effective types of clutter filtration. That is, the silent radars range and speed gating functions remove clutter in all situations except those in which the clutter has both the same range and speed as the target. Clutter at other ranges will not correlate properly with the silent radar""s internally delayed random pulse signal. This feature is particularly significant in eliminating the clutter which enters the antenna sidelobes of an airborne doppler radar because of the aircraft""s own motion.
Reduced EGM Vulnerability Since the silent radar system will employ range and speed gating and transmits broadband noise at low levels of energy per cycle, it has certain anti-ECM features.
Another advantage of the inventive radar system is since the signal transmitted is a broadband noise signal having a random spectrum even if it were received on a target intercept receiver, it does not look like a radar signal. Both manual and automative recognition equipment,is quite ineffective at the intercept receiver even when positive signal-to-noise ratios are achieved. In normal operation, the signal received by the target""s receiver will be below this signal-to-noise threshold.
Still another benefit of the invention is that most intercept receivers pick up the radar sidelobes rather than the main beam because of the greater number of sidelobes and therefore the increased probability is that the target will be illuminated by the sidelobes rather than the main beam. Since normal radar sidelobes are xe2x88x9220 to xe2x88x9230 db down from the main beam and the same holds true of the invention, the signal from the invention would not be able to be intercepted in the radar sidelobe region even if the signal in the main beam was strong enough to be detected.
Against repeater type jammers, the silent radar has greater invulnerability to range deception since range gates are extremely narrow as a result of the very wide spectrum transmitted. As repeater type jammers have inherent minimum delays which are greater than the equivalent range resolution of this radar, their false signals can only appear in gates at ranges greater than the skin echo of the target, and therefore can be ignored. Even if the false doppler signals are introduced into a silent radar by a repeater jammer employing frequency translation techniques, they will always appear at a greater target range than the true echo.
CW jammers can be eliminated with tracking rejection filters with negligible degradation to the reflected target signal.
Repeater type jammers in friendly aircraft could be used to amplify and repeat the silent radar signal even though this signal""s power level is extremely low (even below the natural noise level in the jammer""s receiving components). Suitable electronic circuitry in the repeater could pulse the amplified repeated radar signal, on and off, in a coded fashion. The return signal from such a repeater, when received by the silent radar system, could be decoded and used to identify the aircraft as friendly.
Or, even more detailed information could be transferred from aircraft to radar. Such information could actually identify the aircraft as to class, squadron, aircraft number, etc. Such coding would not reduce the effectiveness of the repeater in its assigned ECM mission and yet would add an important information collection capability to silent radar. The aircraft carrying the repeater would never know it had been xe2x80x9cseenxe2x80x9d by the silent radar, but the information would still be automatically exchanged.
It is a principal object of this invention to provide a silent radar whose transmission cannot be identified by an approaching intercept receiver.
It is another object of this invention to provide a silent radar that has a variable output power system for the detection of targets without itself being detected.
Another object of this invention is to provide a radar system which suppresses clutter to a minimum.
Another object of this invention is to provide a radar system that obtains unambiguous range and velocity information of a target or targets.
Another object of this invention is to provide a silent radar system which employs a variable coded or uncoded frequency spread (modulation sweep or pulse compression) together with random or non-random phase reversals controlled by a pulse signal to generate wideband random noise spectra which can be remembered by narrower band delay devices.
Another object of this invention is to steer wide noise transmission into crowded portions of the electromagnetic spectrum where other signals will mask the radar signal and thereby provide a silent radar.
Another object of this invention is to provide an unambiguous radar that eliminates ambiguities in range and velocity by utilizing the transmission of wide band random noise and correlation techniques.